502 Prof. Sedgwick on the May Hill Sandstone, 



I believe the Director of the Government Survey thought that 

 I had abandoned my old nomenclature (and well might he think 

 so after what I have stated respecting the tampering with the 

 names of my reduced map), and that the Silurian nomenclature 

 was to be taken as a fact established ; and hence, that the primary 

 object of the Surveyors was to follow out, and give meaning to, 

 the Lower Silurian groups. Be this as it may, they did not 

 keep their nomenclature in abeyance till they had worked out all 

 the physical and palaeontological subdivisions of the Cambrian 

 series ; and hence their successive groups have a very great geo- 

 graphical inconsistency, and were not all palseontologically right, 



Thirdly. The Middle Silurian group of the Survey has lost its 

 meaning since the establishment of the May Hill group. On 

 this enough has been said before. The May Hill group is, in 

 the nomenclature of this paper, not an Upper Cambrian, but a 

 true Silurian group. It cannot therefore be called Upper Cara- 

 doc without the great inconvenience of leading us to a false 

 association, and also to a locality where the section does not give 

 us a good May Hill type. 



Though the great Cambrian section (which is in the first 

 place derived from the beds on the west side of the Carnarvon 

 chain, and ascends to the Harlech grits, and is afterwards taken 

 up at the representative of the same grits near the Merioneth 

 anticlinal, and is thence carried across the Berwyns) is not geo- 

 graphically, yet it is geologically, continuous. The section on 

 the east side of the great Arenig gives us a beautiful and uninter- 

 rupted Bala series, which we do not see in the Upper Cambrian 

 groups of South Wales : yet the Bala series is defective at the 

 top, where it is overlaid, I believe unconformabiy, by the May Hill 

 sandstone. In this respect the Upper Cambrian sections of South 

 Wales are not only, I think, thicker^ but much more perfect in the 

 ascending order than those of North Wales ; and this fact, if I 

 rightly interpret it, is explained by those movements of elevation 

 which produced among other phsenomena the conglomerates and 

 coarse sandstone on the eastern skirts of the Radnor mountains; 

 and were succeeded by the unconformable deposits at the base 

 of the Upper or true Silurian rocks. 



When I crossed South Wales in 1 846, I still thought it pos- 

 sible (giving it a very broad margin) to construct a Cambro^Silu^ 

 rian group which should include the Llandeilo flag, and yet 

 leave sevei'd older slate groups to the west of it, before we 

 reached the shores of Cardigan Bay, or the flank of Ca<der Idris. 

 I now abandon that hypothesis ; and since the establishment of 

 the May hill group, the name Cambro-Silurian ia without my 

 volue or geological meaning. 



To icall ail the contorted old rocks to the east of Cardigan Bajr 



